Thats unfortunate, however a release has already been made with the changed ABI, so any changes now would only make it worse. And master had a major bump to resolve any ABI concerns.

It's not the first time this happened. See 3.1 and lavfi.
vector_dmac_scalar() is only available in release/3.4 fortunately, so it should be moved down to the end of the struct for 3.4.1 and the release news entry have a line about it, just like we did with 3.1.1.

The major bump doesn't really solve this for those trying to stay compatible with the 3.x ABI, which means every 3.4 user, the latest stable release.

Thats unfortunate, however a release has already been made with the changed ABI, so any changes now would only make it worse. And master had a major bump to resolve any ABI concerns.

It's not the first time this happened. See 3.1 and lavfi.
vector_dmac_scalar() is only available in release/3.4 fortunately, so it should be moved down to the end of the struct for 3.4.1 and the release news entry have a line about it, just like we did with 3.1.1

Personally I think making 3.4 and 3.4.1 ABI incompatible is worse then just keeping it as-is.
Its not a user-facing ABI break, its only inter-library. A strong recommendation to always upgrade all libraries should always be present on every release.